We have demonstrated that luteinizing hormone-releasing hormone (LHRH) and galanin are colocalized in a subset of hypothalamic LHRH neurons and the degree of colocalization is estrogen-dependent. In the female rat hypothalamus, the number of LHRH cells coexpressing galanin is four to five-fold higher than in the male. In pregnant, lactating or old animals the degree of colocalization is similar to that seen in the male animals indicating that progesterone, the level of which is higher in each of these animal models, can blunt the effect of estrogen on galanin gene expression. Following treatment of lactating animals with RU-486, a potent progesterone receptor antagonist, the incidence of colocalization of LHRH and galanin increased supporting further the blunting effect of progesterone. Our observations also indicate that galanin can be considered as a marker for sexual differentiation, since in male postnatally-castrated animals, whose hypothalami were masculinized by sexual steroids, estradiol induces galanin gene expression if administered in adulthood. Our functional data indicate that galanin regulates reproductive functions at both the hypothalamic and pituitary levels.